Ammonia synthesis catalyst and method of making same



UNITED STATES ra'rrzrrr OFFICE.

JOHN OOLLINSCLANCY, OF NIAGARA FALLS, NEW YORK, ASSIGN OR TO THENITROGEN CORPORATION, OF PROVIDENCE, RHODE ISLAND, A CORPORATION OFRHODE ISLAND.

AMMONIA SYNTHESIS CATALYST AND METHOD OF MAKING SAME.

No Drawing. Application filed June 25,

To all whom it concern Be it known that I, JOHN COLLINS CLANoY, acitizen of the United States, residing at Niagara Falls, in the countyof Niagara and State ofNew York, have invented certain new and usefulImprovements in Ammonia Synthesis Catalysts and Methods of Making Same,of which the following is a specification.

This invention relates to a process of synthesizing ammonia from itselements and in one of its aspects is especially concerned with certaindiscoveries .which 'I have made in connection with catalytic materialsby means of which such synthesis may. be effected.

Much has beenwritten, especially in patent literature, concerning thesupposed eflicacy of various catalysts for the synthesis of ammonia; andamong these calcium cyanamid ismentioned in the French patent to Brochetand Boiteau, 'No.;425,952, published June 24, 1911.'

Calcium cyanamid is therein described as a suitable catalyst and thespecific statement is made that the combination of hydrogen and nitrogeneffects itself at atmospheric pressure, but one can equally operate witha different pressure.

I myself, long since arrived at the conclusion that such a substance ascalcium cyanamid should be capable of being effectively used as acatalyst for the purpose in'qu'estion; but subsequently discovered, whenat tempting touse it under pressures approximating that ofitheatmosphere or somewhat higher, that that catalyst was not a truecatalyst, since it decomposed when heated to operating temperatures (6.9. 450 (1.), when in the presence of a mixture of hydrogen andnitrogen,ammonia and hydrocyanic acid bein liberated.

This dlscovery for a time? caused me to relinquish all thought of.'using calcium cyanamid as an ammonia synthesizing catalyst; but Isubsequently made a further discovery, which was in a way mostunexpected,

namely, that when relatively high pressures are used in the synthesizingoperation, this catalyst, especially when prepared ashereinafter'described, is not destroyed,no hydro.-

cyanic acid being liberated.

In other words, properly prepared calcium cyanamid, when used tosynthesize ammonia Specification of Letters Patent.

1918. Serial No. 241,758.

from a mixture of the elements of the latter, preferably in combiningproportions, at a temperature which ordinarily would cause dissociationof the catalyst, but under a high pressure,does not dissociate, butrather acts as a true catalyst and moreover a particularly effe ctiveone..

This discovery is directly at variance with the statement in the Frenchpatent aforesaid; and constitutes, I believe, a material step forward inthis art.

That the mere application of pressure'to the gaseous mixture beingtreated, whereby in turn to subject the exceedingly porous and otherwiseunstable catalyst material to high pressure, should thus apparentlyrender this cyanamid stable and enable it to act as a true catalyst, isnoteworthy, and assuredly not to be expected from any disclosure in theart with which I am acquainted.

I have also discovered that calcium cyanamidis not the only substancewhich behaves in this peculiar fashion. Thus,--to select anothercyanamid the metal basev of which belongs to a totally different groupof .Inetals,copper cyanamid, while of itself not an efficientlow-temperature catalyst, nevertheless behaves in a very similar way, inthat when properly prepared for use in a catalyst and subjected toammonia synhtesizing conditions, using a gaseous mixture of nitrogen andhydrogen under say 1500 pounds pressure, it is not decomposed; while onthe other hand, with everything else the same as in the precedingexample, except that'the pressure used is but substantially that of theatmosphere,-I find that the copper cyanamid behaves just as does thecalcium cyanamid under such circumstances, namely, it decomposes withliberation of hydrocyanic acid.

used under high pressure is available for use in a low-temperaturecatalyst, especially as a carrier for more active catalytic material, as

hereinafter described.

Copper cyanamid is especially worthy of remark; because copper has, Ibelieve, generally been regarded by those familiar with this art,,as acatalytic poison. j

Again, barium cyanamid is another substance which is converted to astable catalyst and prevented from decomposing during Patented'Sept. 7,1920.

pheres pressure.

catalyst for the synthetic reaction in ques-- tion.

As is well known, the function of a catalyst of the kind in question isto promote or expedite a reaction which, if given sufiicient time, wouldcontinue to take place in any 'event, if not otherwise interrupted,until equilibrium under the prescribed conditions had been established;and, loosely speaking, there are an enormous number of substances whichcan perform the function, to some measure, of speeding up thecombination of mixed nitrogen and hydrogen, to form ammonia. r Themeasure of the efliciency of such catalysts is however in nearly everyinstance very small indeed, and in most cases is negligible; certainlyfrom any practical or commercial standpoint.

Further, and aside. from the chemical composition of the substanceselected, I'have found that the physical condition of the oontact bodyused, is next in importance to chemical composition, for the purpose inquestion. Thus calcium cyanamid as prepared commercially is, even whenused under high pressure, by no means comparable as catalytic materialto this substance I as prepared according .to the methods hereinafterdescribed.

Properly speaking, then, commercial cal- I cium cyanamid, in the absenceof some preparatory treatment to open it up, so to speak, and render itporous, is not specially good catalytic material for commercialammoniasynthesis; because it is not in physical condition to adaptit for usefor that purpose. It is too dense and hardand offers for a given weightthereof, but comparatively little surface in contact with which the synthesis can be effected; and to render this and like substances availablefor use, they must be produced or treated in a manner to impart to themphysical characteristics quite different from the familiar ones thereof.My new catalytic materials may convenlenetly be prepared in variousways, but in any case are preferably formed at relatively lowtemperatures; e. g. a black heat, and, deslrably, very much lower,indeed, than this. For example, I may prepare a solution of commercialcalcium cyanamid in water and purify it with nitrate of silver, toeliminate the sulfur. The solution filtered off from yield crystals ofthe silver sulfid, is one of pure CaCN This solution is now treated withcarbon dioxid gas, and, after the calcium carbonate formed has beenseparated by filtration, the filtrate is evaporated to dryness undervacuum, to

c anamid (H CN These crystals, I have discovered, are soluble in liquidammonia, and to such solution I add pure metallic calcium, which alsoissoluble in liquid ammonia; the calcium' being added in molecularproportions, to form CaCN Obviously, this reaction is preferablyeffected at a temperature of zero egrees centigrade, or lower, and theresultant precipitate is a pure white calcium cyanamid, much like sugarin appearance, except that it is exceedingly fiocculent and porous.

It may be readily separated from the liquid ammonia by filtration andany of the solvent remaining therein may, of course, readily be drivenoff as vapor at a low temperature.

The so produced calcium cyanamid is white and exceedingly porous andopen,

whereas commercial cyanamid, usually produced in an electric furnace andat high temperatures, is black and dense; a sintered roduct, unsuitablefor use 'as a catalyst.

oreover, when calcium cyanamid is dissolved in water and evaporated, itdissociates into dicyandiamid and urea. ommercial calcium cyanamid hencecannot well be purified in this manner, and while it is probable that itcould be produced relav tively quite pure, in the electric furnace, itwould be quite too dense for the purpose intended.

Barium cyanamid, the second alkaline earth metal cyanamid, mentioned byway of illustration in the foregoing, may be produced in form to behaveas an active catalyst, as follows: I Barium cyanid is prepared in purecondition and is treated in a closed .receptacle with nitrogen orammonia vapor, while heated to a temperature preferably less thanv blackheat; say 350400 O. or slightly lower. Barium cyanamid,BaCN is formed,brown or black in color and peculiarly expanded and porous in character.The color 1s apparently due to free carbon in the product, At about 350C. the mas sturns light brown; at about 375 C. it becomes dark brown;and" as 400 C. is approached, it turns black. It also tends to darken atthe lower temperatures noted, as the period of treatment is prolonged.

Lumps of this material, which are quite self sustaining in character,may be used under pressure (to revent decomposition with formation ofCN, as noted), as efficient catalysts for ammonia synthesis. Expandedand highly porous strontium cyanamid may be similarly prepared.

In fact, it is preferable to heat piecemeal, that is to say not in mass,as the mass if of considerable size is apt to explode violently. Thisheating operation may be conducted in an atmosphere of ammonia vapor, ornitrogen and hydrogen, or nitrogen, or in any inert gas; and the smallportions being dried will usually pop or sputter as the gas generated inthem expands and swells them out to form a highly expanded, poroussubstance, which is then ready for use as a catalyst. f I

Another mode of preparing the copper cyanamid catalytic material is toconvert calcium cyanamid into ammonium cyanamid according to theequations:

In using either ammonium carbonate or oxalate, as per these equations,it Wlll 'be understood that such materials, as also the calciumcyanamid, are preferably chemically pure. The insoluble lime carbonateor oxalate formed is filtered off and to the filtrate I add a solutionof copper chlorid, to form a jet black precipitate of CuCN This isseparated from theliquid by filtering and is washed to free it fromchlorids. It is then dried and treated as before to form the highlyexpanded material.

The porous lumps of pure copper cyanamid'thus formed may be converted tocopper nitrid by treatment with nitrogen mixed with hydrogen, at 500 C.or some- .erly characterized as derivatives of cyanamid.

The preparation of silver compounds corresponding to the above coppercompounds may readily and similarly be effected by sub- 'stituting asuitable silver salt, such as the nitrate, for the corresponding coppersalt.

It is also possible to combine the above mentioned materials toadvantage in certain ways. Thus, if after the masses or lumpsof, forexample, expanded copper cyanamid have been prepared, they are. thentreated with a solution ofmetallic calcium, or the like, in liquidammonia and the latter is vaporized ofl",-there will have been formed anexpanded mass of copper cyanamid, impregnated throughout itsmultitudinous pores with metallic calcium. All of the foregoing opeations are carried out in closed receptac es and in such fashion as toavoid contamination by oxygen from the air or from other sources. I h vThe so formed impregnated copper cCyanamid is then heated to, say,250-300 in an autoclave or elsewhere, with, however, no oxygenpresent,to form calcium cyanamid in particularly active catalyticcondition, directly in the pores of the expanded copper cyanamid lumps,which now will also contain metallic copper. This metal is present, ineffect, in ramified form and acts as a conductorof heat during thesubsequent exothermic, ammonia-forming reaction.

Silver may, of course, be substituted for copper in the above, asalready observed.

I desire notto be limited to the above recited modes of preparingmaterials in condition for use as effective catalysts for ammoniasynthesis; those-herein described being given merely by way of example.Thus, by way of further illustration: 1

Starting with a solution of pure metallic calcium in liquid ammonia, Ibubble hydrocyanic acid gas through said solution to form calciumcyanid, Ca(CN) whlch 1s precipitated as a flocculent white powder,

the particles or crystals of which are finer than those of theanalogously produced calcium cyanamid. k

This precipitate is readil separated from the liquid ammonia by fitration and responds to all of the tests for acyanid and not to thosefor a cyanamid. This substance, so far as I am aware, has never beforemy discoverv of this mode of preparing it at a very low temperature,been produced in solid but porous and flocculent form. Never, before,however, has it been produced by means of such a menstruum as liquidammonia, which possibly accounts for this.

This novel material is particularly available for use in the formationof a catalyst for the synthesis of ammonia from its elements, whenoperating under pressure, noted.

WVhen placed in the catalytic chamber of a suitably constructedapparatus and subjected to a mixture of pure hydrogen and nitrogen, incombining proportions and under pressure, this material or rather amodification threof,-since the catalyzing subtion appears to'-substantially cease aftera time.

.Here then is acatalyst of the character in question which requires buta comparatively low pressure, 6. g. twenty-five atmospheres or lower tostabilize it practically from the beginning of its use under operatlveconditions and it is produced at around 200 C., so that it permits ofits employment as an effective catalyst at a temperature of from 300-350 0., and, if desired, under quite reasonable pressures. This catalyst,by the way, although derived from a cyanid, responds to thecharacteristic tests for a cyanamid, after its production in manneraforesaid.

The factors which I believe are of importance in catalysts for thepurpose in question, if they are to be really effective, may now bebriefly enumerated. p

1. They preferably comprise carbon directly united to nitrogen by aplurality of bonds; although in some instances, as noted, one of theseelements may be absent,--at least initially.

2. They should preferably be open or porous in structure, whethersupported on a peltirrier or in the form of lumps, or the 3. They shouldbe produced at a low temperature, preferably in no case above a blackheat and desirably much lower.

4. They should be of such a nature that they tend to dissociate to someextent at least, in the presence of the gases being treated, and at thetemperature at which they ,are used, if not subjected to sufiicientpressure to overcome said tendency and render them stable while yetleaving them extremely active catalytically.

. The tendency to dissociate (and its prevention) in the observedmanner, appears to be of great importance'and its value in a catalystfor ammonia synthesis has apparently never been noted by any of thenumerous investigators in this art. By virtue of this tendency, thecatalytic material is so active that its constituents are able toevanescently combine with one or both of the gases being treated,probably to afford the continuous delivery in the pores of the catalystof said gas or gases in nascent condidepend perature,

tion or possibly in actual combination as ammonia. v

Further, so far as I am aware, no one seems to have realized that suchsubstances which thus tend to dissociate at insufliciently highpressures when under the remaining requisite conditions, can be renderedjust sufliciently stable for the intended purpose by subjecting them toa higher pressure. As to the pressure required, that of course will uponthe catalytic material used, as above indicated. It will also dependupon the temperature of the operation; and mally the lower thetemperature. at which the synthesis can be effected, the better, forseveral reasons. i

In the first place it is well recognized that the higher thetemperature, the less the ammonia content that can exist in a mixture ofnitrogen, hydrogen and ammonia, assuming equilibrium to have beenestablished. Conversely, then, with a really efficient catalyst for lowtemperature synthesis, the possible yield of ammonia increases as thetem erature is lowered toward, say, 250300 Secondly, the employment ofsuch low temperatures obviates substantially all of the tremendousdifficulties attendant upon the use of high pressures with suitable andnot prohibitively costly apparatus, at temperatures which range around450 C. or more.

In this disclosure it is my desire to point out those factors concerningcatalysts for ammonia synthesis which appear to have 1 been overlooked,and inthis connection I may state that while,

thus far, ,I have considered, by way of example, only having a metallicbase, 0. 9. Ga

- which will not readily polymerize has been driven off, the material,which is normally nor- substances CN ,it is I by no means essential thatthe catalyst be in the form of an expanded porous-mass, is

then available for use as a catalyst for the usual mixture of nitrogenand hydrogen, preferably in combining proportions The synthesis may beconducted at a low temsuch as is hereinbefore referred to, and ispreferably effected under pressure to with certainty prevent furtherdecomposition of the catalyst, which of course is to be avoidedwith thisor any other hydrocyanicacid yielding substance,

The silver, left in somewhat connected formation in the catalytic mass,seems to be principally of value as a heat conducting medium to aid inequalizing the temperature of the catalyst throughout, when in use. Theammonia forming reaction is of course exothermic but proper control ofthe temperature of the incoming gas mixture in conjunction with the heatconductive metal in and about the catalyst, afl'ords means to preventimpairment of the process by possible undue rise in temperature in theinterior parts of the catalytic mass or masses when the latter arerelatively large.

It is, of course, by no means essential that the paracyanogen beproduced as above described. Also, since it is soluble in concentratedsulfuric acid in the cold, it may by this means be introduced into thepores of a suitable support, 6. g. pumice,to be precipitated upon thewalls of said pores by dilution with water. As the precipitate isinsoluble in water, it may then be thoroughly washed free from the acidand dried, preparatory to use in manner aforesaid.

I am also aware of a number of other similar catalytic substances inwhich the metals are conspicuous by their absence, this important lineof catalysts being one which has apparently received no considerationfrom investigators heretofore; but it is believed that the above examplewill suflice in an already, necessarily rather voluminous disclosure.

To exemplify how efficiently ammonia synthesis may be conducted by meansof my invention, it may be here noted that by the use, for example, ofcalcium cyanamid produced at a low temperature, as above described, asthe catalyst, and when operating at a temperature of even as high as 4250., under somewhat less than one hundred atmospheres pressure, I obtain19 volumeper cent. of NH with the gaseeous mixture of hydrogen andnitrogen in combining proportions, fiowing through the catalyst at therate of 30 liters per hour.

This yield can, of course, be materially raised by elevating thepressure, and especially if the temperature be somewhat lowered while sodoing.

In general, I may add that I prefer to raise the pressure as thetemperature is lowered, since the inexpensive apparatus employed is thenof course better able to withstand the higher pressure.

So far as I am aware no one has heretofore actually effected thesynthesis under pressure of more than possibly a trace of ammonia, whenoperating-at temperatures even as low as 300 0., and I believe this tobe possible only when such extremely active catalysts as those abovedescribed are employed. Obviously, at the lower tempera tures noted,while the equilibriumpercentage of ammonia is higher than for moreeletemperature, is relatively unstable'at such temperature when apressure is used which is, say, a few atmospheres below that actuallyemployed in the synthesizing operation. In other words, it is possibleto over stabilize a given catalyst by the use of an excessive pressurefor such particular catalyst, to in large measure offset the gain-fromthe increase in pressure, principally by rendering the catalyst lesssensitive or efficient.

The temperature at which a given catalyst is or can be produced, usuallyseems to have a bearing upon the point as to the temperature at whichsuch catalyst is likely to begin to evince instability at a givenpressure. Finally, this brings me to a point upon which I desire to laysome emphasis. Heretofore, inventors and investigators appear to'haveattempted-to employcatalysts pro-.

duced at temperatures above those at which such catalysts are used. Inmost instances such catalysts are then altogether too stable to be evenmoderately efficient. I

I prefer, on the contrary, to actually produce the catalytic material,per 86, at a temperature below the synthesizing temperature, or even,insome cases materially below the temperature at which thea nmonia is tobe formed,the latter cases, more especially, where very high pressuresare 'subsequefitly employed during the synthes1s, to' stabilize suchcatalysts. v

This procedure I believe involves a distinct departure in the-art.

Having thus described my invention what I claim is:

1. As an article of manufacture, a catalyst for the synthesis of ammoniafrom its elements, comprising a cyanamid compound in highly porous'form.

2. As an article of manufacture, an effective catalyst for the synthesisof ammonia from its elements, comprising an inherent] porous nitrogenouscatalytic mahe surface of which is free from the terial t densifyingeffect thereupon of any previously applied sintering heat.

3. As an article of manufacture, an effective catalyst for the synthesisofammonia from its elements, comprising a highly porouscarbo-nitrogenous catalytic material the surface of which is free fromthe densifying effect thereupon of any previouslyapplied sintering heat.

4. As an article of manufacture, an effective catalyst for the synthesisof ammonia from its elements, comprising a material the surface of whichis free from the densifying effect thereupon of any previously appliedsintering heat.

5. As an article of manufacture, a flocculent compound of calciumcomprising carbon united to nitrogen by a plurality of bonds.

6. An eflicient catalyst for ammonia synthesis, which comprises asulfur-free cyanogen compound of calcium in highly porous condition,said compound being active catalytically and the surface thereof beingfree from the densifying effect thereupon ofany previously appliedsintering heat.

7 An eflicient catalyst for ammonia synthesis, which comprises asulfur-free cyanogen compound of an alkaline earthmetal, in highlyporous condition, said compound being active catalytically and thesurface thereof being free from the densifying effect thereupon of anypreviously applied sintering heat.

8. An efficient catalyst for ammonia syn thesis, which comprisessulfur-free cyanamid of calcium originally formed and subsequentlyhandled at all times at temperatures which are insufficiently high topermit densification of its surface by sintermg.

9. The method of obtaining an eflicient catalyst for ammonia synthesis,which comprises producing a carbo-nitrogenous material for said catalystat a low temperature in par icularly reactive condition through theintermediacy of liquid ammonia as a solvent for a constituent of saidmaterial.'

10. The method'of obtaining an eflicient of any catalyst for ammoniasynthesis, which comprises producing a carbo-nitrogenous material forsaid catalyst in particularly reactive condition through the'intermediacyv of liquid ammonia as a solvent for a constituent of saidmaterial, volatilizing off said liquid-ammonia to recover the materialpreviously dissolvedtherein, and reacting upon the recovered material ata temperature below the sintering point thereof to produce a catalyticsubstance the surface of which is free from the densifying efi'ectthereupon previously applied sintering heat. 11. The method of obtainingan eflicient catalyst for ammonia synthesis, which comprises producingcyanid of calcium through the intermediacy of liquid ammonia,volatilizing off said liquid ammonia to recover said cyanid of calciumin solid form, and reacting upon said calcium cyanid to produce cyanamidof calcium at a temperature below the sintering point of the latter material.

12. The method of obtaining an eflicient catalyst for ammonia synthesis,which comprises producing cyanid of calcium through the intermediacy ofliquid ammonia, volatilizing off said liquid ammonia to recover saidcyanid of calcium in solid form, and reacting upon said calcium cyanidto produce cyanamid of calcium at a temperature below 400 C. r

In testimony whereof I have' affixed my signature in the presence of twowitnesses. JOHN COLLINS OLANCY.

Witnesses;

FRANCIS G. SMITH, RANsoM'O. CROWLEY.

